The analysis of samples and the determination of the presence or concentration of chemical species contained therein is a common and important process in chemistry and biology. Particularly important is the analysis of biological fluids, such as blood, urine, or saliva, to determine the concentration of various constituents. Also of great importance is the measurement of the concentration of various chemical constituents embedded within biological materials, such as tissue. Chemical analysis of blood, urine, and other biological fluids is crucial to the diagnosis, management, treatment, and care of a wide variety of diseases and medical conditions. In the case of diabetes, monitoring of blood glucose levels several times a day is necessary to the efficient management of this disease in many patients. Analysis of various blood components is of importance in both the diagnosis and treatment of diseases of the circulatory system. For example, the level of various types of cholesterol in the blood has a strong correlation with the onset of heart disease. Urine analysis provides valuable information relating to kidney function and kidney disease. The concentration of alcohol in the blood is known to be related to a subject's physical response time and coordination and can provide information related to, for example, the individual's fitness to drive a motor vehicle.
Additionally, there are many instances where it is desirable to measure the local concentration of chemical constituents in tissue, either in-vivo or in-vitro. For example, in stroke victims it is important to monitor the degree of brain edema or the concentration of various metabolic chemical constituents in the brain that serve as indicators of brain function. Such indicators include fatty acid compounds, water, blood, lactates, and certain proteins and lipids. Other specific examples may include the monitoring of tissue oxygenation or tissue blood perfusion as a means to of gauging the metabolic function of a human or animal subject.
Moreover, in many applications, a “real-time” measurement of chemical concentration in biological fluids is important Current invasive methods require that a sample of fluid be removed from a subject and then analyzed in one or more chemical tests. The tests can be expensive and require skilled technicians to remove and analyze the samples. Furthermore, the analysis of samples may have an undesirably long turn-around time. Additionally, the tests are usually made in centralized clinical laboratories with a resulting complexity of sample tracking and quality control. These circumstances create additional problems related to the potential change in the chemical composition of the fluid between extraction and analysis and, even more detrimentally, the possibility of a sample being confused with the samples of other patients.
It is also advantageous to analyze the chemical nature of sample materials without physically extracting a sample from the subject. For example, it is advantageous to examine the chemical makeup of human blood without taking a blood sample. In addition to time and cost considerations such invasive testing causes skin trauma, pain, and generates blood waste.
For all of the foregoing reasons methods of “non-invasive” testing have long been considered an attractive alternative to invasive testing. However, prior non-invasive testing methods have suffered from a number of practical drawbacks. The present invention is a method of analytical and quantitative testing for the presence of chemical species in a test sample. The method is non-invasive and has wide utility, being easily applicable to the non-invasive measurement of humans, animals, plants, or even packaged materials. Being highly versatile the method is broadly applicable to both in-vivo and in-vitro samples.
1. Brief Description of the Related Art
The concept of non-invasive testing is not unknown in the art. What has been elusive is the ability of quickly, easily, cheaply and accurately conducting measurements.
Certain infrared (IR) detection techniques are known and have been used to detect the presence of chemical constituents in the blood. Specific examples include the IR detection of oxygen saturation, nitrous oxide concentration, carbon dioxide concentration, or measurement of oxidative metabolism, and blood glucose levels. The goal of these inventions is the determination of human blood chemistry. A typical present technology projects light into the body while measuring the light after it passes through the body. Comparing the input beam with an exit beam allows a rough determination of blood chemistry. Unfortunately, these techniques suffer from a number of inadequacies, most especially, tissue interference, lack of specificity, and limited accuracy. A number of prior art patents describing such techniques are set forth below.
Kaiser describes, in Swiss Patent No. 612,271, a technique for using an IR laser as a radiation source to measure glucose concentrations in a measuring cell. This technique uses venous blood passed through extra-corporeal cuvettes at high blood flow rates. This has the undesirable effect of heating the blood and requiring that the blood be removed from the patient's body. Kaiser does not describe a non-invasive technique for measuring glucose concentration.
March, in U.S. Pat. No. 3,958,560, describes a “noninvasive” automatic glucose sensor system which projects polarized IR light into the cornea of the eye. A sensor detects the rotation of this polarized IR light as it passes between the eyelid and the cornea. The rotation of polarized light is correlated to glucose concentration. Although this technique does not require the withdrawal of blood, and is thus, “noninvasive”, the device may cause considerable discomfort to the patient due to the need to place it on the patient's eye. Furthermore, March does not use an induced temperature gradient or absorbance spectroscopy as does the present invention. As a result, the present invention involves no physical discomfort and is more accurate.
Hutchinson, in U.S. Pat. No. 5,009,230, describes a glucose monitor which uses polarized IR light to non-invasively detect glucose concentration in a person's blood stream, The method requires an external IR source, which is passed through a portion of the human body. However, the accuracy of measurement is limited by the wavelengths of the polarized light beam (940-1000 nm) being used. Unlike the present invention, Hutchinson relies on detected changes in the polarization of the incident light beam. Furthermore, Hutchinson does not use an induced temperature gradient as does the present invention.
Similar limitations are found in Daline, et al., in U.S. Pat. No. 4,655,225, which describes a similar spectrophotometric technique. Dahne uses a directional external IR radiation source to emit a beam. Reflected and transmitted light from the beam is used to determine the glucose concentration Daline differs from other techniques in using radiation at wavelengths between 1000-2500 nm. Unlike Dahne, the present invention is not confined to using wavelengths, between 1000-2500 nm. Dahne also does not use an induced temperature gradient as does the present invention.
Mendelson, et al., in U.S. Pat. No. 5,137,023, uses a different concept known as pulsatile photoplethysmography to detect blood analyte concentration. The instrument of Mendelson is based on the principles of light transmission and reflection photoplethysmography, whereby analyte measurements are made by analyzing either the differences or ratios between two different IR radiation sources that are transmitted through an appendage or reflected from tissue surface before or after blood volume change occurs in response to systolic and diastolic phases of the cardiac cycle. Once again, the technique requires the use of external IR sources and is susceptible to interference from body tissue and other blood compounds.
Rosenthal, et at., in U.S. Pat. No. 5,028,787, discloses a non-invasive blood glucose monitor which also uses IR energy in the near IR range (600-1100 nm) to measure glucose. As with the above-mentioned devices, these wavelengths suffer from poor analyte absorption which results in poor resolution and insufficient specificity.
Cho, et al., in PCT No. PCT/DE95/00864, discloses a blood glucose monitor which uses heat flux generated in a patients fingertip to measure metabolic rate. Indirectly, this approximation of metabolic rate is used to measure approximate glucose concentration.
Major steps forward are embodied in the glucose measuring techniques disclosed in the patents to Braig, et al., U.S. Pat. No. 5,313,941 ('941). However, the '941 patent requires an independent external IR source to determine blood analyte concentration.
Optiscan, Inc of Alameda, Calif. has expanded the concept of gradient absorbance spectroscopy and demonstrated the utility of non-invasively measuring differential absorbance to determine blood glucose concentration in human subjects in U.S. patent application Ser. Nos. 08/816,723 and 08/820,378, both of which are hereby incorporated by reference.
2. Scientific Background of the Invention.
An understanding of the present invention requires an understanding of the concepts of transmission spectroscopy and gradient spectroscopy.
Basic transmission spectroscopy identifies analytes (an analyte is defined as a chemical species sought to be identified by the present invention) by comparing a light beam passed through a test sample to a reference beam not passed through the sample. Typically, transmission spectroscopy requires the test sample be removed from its native environment to a sample holder for analysis. The absorbance spectrum of the sample is examined. At specific wavelengths (known as analyte absorbance peaks) the light from the beams are compared. By using Beer's Law and comparing the sample beam with the reference beam in selected absorbance regions the absorbance of a sample may be measured and a determination of analyte concentration may be made. This is known as classical transmission cell spectroscopy. Strictly speaking, this method is unsuitable for non-invasive measurement. Significant problems being the need for extracting samples and the inability to accurately determine the pathlength of the beams used to analyze in-vivo samples. Progress has been made in overcoming these limitations as shown in the patents to Braig, et al., in U.S. Pat. Nos. 5,313,941, 5,515,847, and 5,615,672 and in U.S. patent applications Ser. Nos. 08/816,723 and 08/820,378. These patents and patent applications have laid the groundwork for the novel advances embodied in the present invention and are hereby incorporated by reference.
An understanding of the radiation emission characteristics of matter are also needed. All objects at a temperature greater than 0 K emit electromagnetic radiation in the form of photons. Ideal blackbody radiators (objects having an emissivity coefficient em=1.0) radiate energy according to the Stefan-Boltzmann Law and Planck's Equation (i.e. radiation output increases with increasing temperature). Additionally, many non-blackbody objects demonstrate near-blackbody radiation characteristics. For example, the human body's spectral radiation characteristics are very similar to that of a blackbody radiator and may be described as a “graybody” distribution (for example, having an en, of about 0.9). These radiative characteristics provide known sources of IR radiation which may be used to non-invasively analyze the constituents of a test sample.
Furthermore, an analysis of radiation behavior shows that, in objects at a constant and uniform temperature, photons emitted from the interior of the object are reabsorbed within 10-20 μm of the point of origin. Thus, an external radiation detector cannot detect radiation emitted from deeper than 20 μm inside an object. Under these conditions, only an object's surface emission spectrum is detectable by a detector. This poses a significant problem for non-invasive measurement techniques seeking to analyze chemical characteristics present deeper within an object.
The field of gradient spectroscopy was developed, in part, in an attempt to overcome the photon reabsorption problem. The Optiscan patent applications Ser. Nos. 08/816,723 and 08/820,378 disclose a thermal gradient, induced by a single temperature event and a measurement of differences in signal magnitude to non-invasively measure human test samples. The Optiscan applications use a temperature gradient induced by a single temperature event to non-invasively determine the glucose concentration in a human test subject by analyzing differences in signal magnitude at selected wavelengths.
Briefly, in the context of the present invention, a temperature gradient exists where the temperature of a material varies according to some arbitrary function, usually related to depth or time or both. For example, if some material is at an initial temperature (e.g., 37° C.) and a surface of the material is cooled to some lower temperature (e.g., 10° C.) a gradient is induced in the material with the cooled surface being at approximately 10° C. and the deeper (and as yet unaffected) regions being at approximately 37° C. A temperature gradient exists between the two extremes.
The total radiation reaching the surface is the sum of weighted radiation coming from all depths below the surface. The weight given to radiation from a given depth depends on the absorbance of the medium between that depth and the surface. At any particular wavelength, this radiation weighting function decrease exponential-like with depth. If the medium is at a uniform temperature, T, then all depths will be radiating reaching the surface will have a Planck spectrum for temperature T. However, if the temperature below the surface is cooler (or hotter) than the surface, then the contribution to the total radiation from this cooler (or hotter) region will have a different spectrum than the surface radiation. Thus, the total radiation will not have a spectrum corresponding to either temperature, but will be something in between, depending on the relative contributions from the regions of different temperature.